Recently, with the advance of mobile appliances such as smart phones, tablet PCs and the like, substrates for displays have been required to become thin or slim. Display windows or front panels of such mobile appliances are generally made of glass or reinforced glass which is a material having excellent mechanical properties. However, glass materials suffer from the disadvantage of making the mobile appliances heavy due to their own weight and being apt to be broken by external impacts.
As an alternative to glass, plastic resin films have been researched. A plastic resin film is suitable for the trend of pursuing lighter and slimmer mobile appliances because it is lightweight and does not easily break. Particularly, a film with high hardness and wear resistance is required. In this regard, it is proposed to have a structure in which the substrate is coated with a hard coating layer.
As a method of improving the surface hardness of a hard coating layer, a method of increasing the thickness of the hard coating layer may be considered. In order to ensure the surface hardness of the hard coating layer to such a degree that glass can be replaced by the hard coating layer, it is necessary to adjust the thickness of the hard coating layer. However, as the thickness of the hard coating layer increases, the surface hardness thereof may become higher, whereas the hard coating layer may be wrinkled or curled due to the curing shrinkage thereof and may be easily cracked or exfoliated. Therefore, it is not easy to put the hard coating layer to practical use.
Recently, there haven been proposed several methods of increasing the hardness of a hard coating film and solving the problem of the hard coating film being cracked or curled by the curing shrinkage thereof.
Korean Patent Application Publication No. 2010-0041992 discloses a hard coating film composition, free of monomers, using a binder resin including ultraviolet-curable polyurethane acrylate oligomers. However, this hard coating film has a pencil hardness of about 3 H, which is not sufficient to alternate glass panels for displays.